The exact cause of schizophrenia is not known. Mental health experts believe that the condition results from a combination of genetic and environmental factors.

Schizophrenia tends to run in families. Although it occurs in about 1 percent of the general population, it occurs in 10 percent of people who have a first-degree relative (parent, brother or sister) with schizophrenia. People with a second-degree relative (aunt, uncle, grandparent or cousin) also develop schizophrenia at higher rates than the general population. People who have a twin with the disorder face the highest risk, with a 40 to 65 percent chance of developing schizophrenia, according to the National Institute of Mental Health (NIMH).

Genes have also been linked to an increased risk of schizophrenia. Genes are located on 23 pairs of chromosomes (rod-shaped structures found in the nucleus of a cell that contain hereditary material). People inherit two copies of each gene, one from each parent. However, scientists believe that each gene may play a small role in the development of the disorder and is not responsible for causing the disease by itself. Also, scientists are not able to predict who will develop the disorder simply by studying genes.

Recent research studies have also found an increased risk of schizophrenia, autism and certain other disorders in children born to a mother or father of advanced age. This risk is attributed to the declining genetic quality of aging eggs and sperm.

Environment has also been identified as playing a role. Possible environmental factors associated with the development of schizophrenia include:

• Exposure to viruses or malnutrition in the womb.
• Complications during birth, such as mild brain damage.
• Toxoplasmosis, an infection with a common parasite carried by cats.
• Psychosocial factors, such as stressful environmental conditions (e.g., malnutrition).

People with schizophrenia have an imbalance of the brain chemicals serotonin and dopamine. These chemicals are neurotransmitters, which allow nerve cells in the brain to send messages to each other. Everything we experience is the result of brain chemistry sending us messages. In schizophrenia, the input from other people and the environment does not result in the same messages that everyone else receives. An imbalance of brain chemicals can lead to hallucinations and paranoia, which respond to medication that corrects the imbalances.